The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art. Moreover, all publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Because of the relatively high value of gas condensate and liquids, oil gas fields are often injected with nitrogen to enhance production, leading to a substantial increase in nitrogen content in the fields over time. Most typically, the initial nitrogen content in the fields is low, at about 1 to 3 mol %, but as the fields mature, and with continuous nitrogen injection, the nitrogen content can increase to as high as 15 to 30 mol %. Such high nitrogen content requires in most cases the use of a nitrogen rejection unit (NRU) to meet pipeline gas inert specifications that are generally at less than 2 mol % for nitrogen.
Variable and high nitrogen content is also problematic where gas is stored underground in gas storage facilities, particularly during withdrawal and injection cycles. When gas demand is low during summer, excess gas must be injected to the gas storage reservoirs, and when demand is high during winter, gas is withdrawn from the reservoirs. During the withdrawal cycle, it has been observed that the nitrogen content of the withdrawn gas varies from 3 mol % to as high as 30 mol %. To address this problem, most operators install a nitrogen rejection unit to produce on-spec pipeline gas.
Most NRUs operate at cryogenic temperature, as low as −300° F., and for this reason, water and CO2 content must be reduced to ppm levels (e.g., 1 ppmv water, 50 ppmv CO2) to so avoid freezing of water and CO2 in the cryogenic section. Additionally, if the natural feed gas contains significant amounts of aromatics, wax formation and associated operating difficulties will arise, which is a well-known problem in LNG liquefaction plants.
Moreover, the nitrogen content for LNG liquefaction must be kept very low, typically at levels of less than 1 mol % as high nitrogen content in LNG increases refrigeration horsepower demand and hence the liquefaction cost. In addition, high nitrogen LNG has higher boiloff rates during storage and transport, resulting in higher product losses. For these reasons, excessive nitrogen should be removed prior to liquefaction to improve LNG economy. Additionally, heavy hydrocarbons in LNG should be removed to meet the Methane Number specification for vehicles (Methane Number is typically 80 or higher), to minimize emissions from higher hydrocarbons.
One method of removing nitrogen from a natural gas stream is processing of the gas in a NRU, and typical configurations comprising two cryogenic columns are described in U.S. Pat. Nos. 8,435,403 and 4,451,275. Two column systems are particularly advantageous when the feed gas contains large amounts of nitrogen, and where the gas is relatively rich (i.e., has substantial content of higher hydrocarbons). The first column generally operates as a pre-fractionation column operating at high pressure, which removes the heavy hydrocarbon as a bottom liquid, while generating a methane and nitrogen overhead that is fractionated in the second column at a lower pressure. Typically, nitrogen content greater than 15% is deemed suitable as feed gas for plants with two column design. However, two column systems are rarely used for lower nitrogen content (3% or less) and would require in most cases nitrogen recycle to avoid excessive methane losses in the nitrogen vent, which in turn, increases the capital and operating cost of the plant.
On the other hand, an NRU may also be configured to operate with a single fractionating column, and exemplary single column systems are described in U.S. Pat. Nos. 5,141,544, and 5,375,422. Single column systems can advantageously operate at low nitrogen content (e.g., as low as 3 mol %) without nitrogen recycle. They are also often less capital intensive due to the lack of a second column, but require substantially more power to operate on high nitrogen gases. Still further, single column systems are typically limited to processing lean gases, and without use of a pre-fractionation column, heavy hydrocarbons will build up in the system. Therefore, external refrigeration may be required. Additionally, most single column systems use an integral reflux condenser, which is limited to low nitrogen content gases with low reflux requirements.
Compounding difficulties in the operation of NRUs is the fact that most known NRU processes fail to operate efficiently on feed gases with a wide range of nitrogen content (e.g., 3 mol % to 50 mol %) and variable hydrocarbon content. Moreover, many of the known NRU processes require additional refrigeration, heat exchangers, and columns in addition to the basic single or two-column configuration.
Thus, although various NRU configurations and methods are known in the art, all or almost all of them suffer from one or more disadvantages, especially where the feed gas has variable nitrogen content over a relatively wide range and further comprises relatively large quantities of heavier hydrocarbons (e.g., C3+). Therefore, there is still a need to provide improved methods and configurations for nitrogen removal.